The present invention relates to novel fluorinated carbons, and more specifically, to improved amorphous, crystalline and glassy or vitreous fluorinated carbons, products made therefrom and methods of manufacture.
Carbons, which includes both amorphous and crystalline types like carbon blacks, lamp black, graphitic and pyrolitic types, to name but a few, find use in a multitude of important and often critical applications in modern technology ranging from motor brushes, lubricants, batteries, fuel cells, plastic refractories, heat exchangers, composites, nuclear generators, resistors, catalyst supports and so on. A major shortcoming, however, in many applications for carbons is often the limited useful life as a result of oxidative degration.
One solution to the problem of oxidative degration has been the direct fluorination of carbon with elemental fluorine. Direct fluorination methods are disclosed by W. O. Teter el al in U.S. Pat. No. 2,786,874 (1957), J. L. Margrave in U.S. Pat. No. 3,674,432 (1972), D. T. Meshri et al in U.S. Pat. No. 3,929,918 (1975) and T. Komo et al in U.S. Pat. No. 3,929,920 (1975).
Fluorographites, for example, prepared by the direct fluorination method are typically hydrophobic, possess high temperature stability, are insoluble in organic solvents and are relatively unreactive, being attacked neither by strong acids nor by alkalies. They may be represented empirically as (CF.sub.x).sub.n where their specific properties depend on the values for x and n. Fluorographites, for instance, in the range of CF.sub.0.5 to CF.sub.1.0 are used in lithium CF.sub.x batteries as positive electrodes. These batteries possess a high energy density of 320 to 470 watt hour/kg, a high open circuit voltage of 2.8 to 3.2 volts, a high working voltage of about 2.6 volts and a long shelf life. However, the useful range of fluorographite batteries prepared by the direct fluorination method is limited because as values for x increase resistivity greatly increases. In fact, the highly fluorinated material CF.sub.1.1 is almost an insulator or nonconductor.
Not only is fluorographite, for example, manufactured by a costly somewhat hazardous process by direct fluorination with fluorine gas, but frequently undergoes degradation in the process. Strong, highly reactive fluorinating agents, such as elemental fluorine, ClF.sub.3, ClF, CoF.sub.3, etc., have a tendency to produce nondiscriminating reactions with carbon molecules even causing fragmentation of edge sites, grain boundaries, dislocations and other surface imperfections. Strong, nonselective fluorinating agents tend to fluorinate olefinic and aromatic carbon to carbon unsaturation sites, adding to the layered planes of benzene rings of the molecule to provide nonconductive carbons.
According to the present invention, it was discovered that "soft" fluorinating agents are more selective in their attack of functional groups, showing less tendency to degrade and fragment edge sites, grain boundaries, etc., than strong fluorinating agents, like elemental fluorine. That is to say, it was found that soft fluorinating agents, like SF.sub.4 will not react with carbon to carbon olefinic bonds in carbon structures, but instead react with carbon-oxygen bonds to replace oxygen with fluorine at edge sites, grain boundaries, dislocations and other surface imperfections, which are also the same sites where oxidative attack of carbons usually occur. Accordingly, it was postulated that if most potentially oxidation sensitive regions of carbon structures could be selectively fluorinated stable graphite and other carbons could be prepared with greatly improved life expectancies while retaining their desired thermal and electrical properties.
A. C. Teter in U.S. Pat. No. 3,340,081 recognized the incidental presence of surface oxygen in commercial carbon blacks. However, Teter failed to recognize the beneficial effect of specifically pretreating carbon blacks before fluorination to first develop most of the sites of potential instability to oxidative corrosion and degradation. Instead, without further oxidative pretreatment, Teter proceeded directly to fluorinate commercial carbon black with SF.sub.4 or other organic sulfur trifluoride fluorinating agent to a maximum level of 7 percent by weight fluorine to prepare reinforcing agents for butyl rubber vulcanizates.
Accordingly, one aspect of the present invention relates to novel fluorinated amorphous and crystalline carbons and methods of manufacturing such carbons which have longer life expectancies, yet materially preserve their desirable thermal and electrical properties. By developing essentially all such potential sites of oxidative corrosion and degradation prior to fluorination, carbons having higher levels of fluorination can be prepared at relatively low cost by less hazardous specific fluorination methods.
Fuel cells require the use of gas (air or oxygen) depolarized electrodes which are comprised of highly sophisticated mixtures of various carbons, catalysts and polymers, along with other additives, and supportive structures which make up a solid composite electrode. The successful operation of a fuel cell electrode is governed by establishing three-phase interface sites: gas (usually oxygen or air), electrolyte solution (often aqueous acid or base) and the solid composite electrode. However, after extended use, depolarized carbon electrodes tend to become oxidized, lose their hydrophobic properties and "flood" when electrolyte penetrates into their porous structures. Electrolyte solution is drawn further into the electrode structure with eventual depletion of the useful three-phase interface sites. Furthermore, once flooding has ocurred the problem is often irreversible and the fuel cell becomes inoperable. This surface oxidation of carbons and tendency to transform from hydrophobic to hydrophilic properties is also a problem in metal air batteries, e.g. zinc-air batteries and in bifunctional air electrodes. The problem is especially severe in bifunctional air electrodes, since in this instance the gas diffusion electrode acts as an oxygen-evolving electrode in the charge step and is further oxidized. To overcome these problems, workers have sought to use more stable carbons like graphite carbons prepared under special conditions at high temperatures. Catalysts have also been incorporated into composite electrode structures to destroy peroxide species, which form to some extent with reduction of the oxygen or air feed, and which accounts for some of the oxidative degradation of carbon surfaces.
Accordingly, a further aspect of the present invention is the preparation of improved fluorinated carbon composites, including fuel cell electrodes, metal-air batteries and bifunctional air electrodes. The improved fluorinated carbons more effectively protect sites on the carbon electrodes which would otherwise be prone to flooding and additional degradation. The present invention also provides a method for regenerating spent-flooded gas diffusion electrodes for reuse by specific fluorination, as described in further detail below.
It has been known for many years that ozone, O.sub.3, could be produced electrochemically using various kinds of anodes, e.g. lead dioxide and platinum, in cold acidic electrolyte solutions, like sulfuric acid and phosphoric acid. In most cases, however, the energy conversion efficiencies were found to be low. More recently, Foller et al reported in CEP. (49-51), Mar. 1985 that ozone could be generated at much higher current efficiencies using vitreous or glassy carbon anodes in a 48 percent aqueous tetrafluoboric acid solution. In spite of the apparent advantages of electrochemical ozone generation with vitreous carbon anodes their exposure to highly corrosive acidic electrolytes has been found to shorten their useful life expectancies, making them a less attractive alternative. Moreover, current efficiencies for ozone are still too low and uneconomical for many applications.
Accordingly, a further aspect of the present invention, relates to the discovery that fluorination of vitreous or glassy carbons results in a material which is considerably more stable to corrosion in cold aqueous acids, including sulfuric, phosphoric and tetrafluoboric acids making such carbons currently more suitable and economical as anodes in the electrochemical generation of ozone as well as for other uses as anodes, such as persulfate formation.